The present invention relates to a new and improved method of transmitting measuring or measurement values in a fire alarm system, wherein the measuring values determined by individual fire alarms located in a series configuration at reporting or signaling lines are delivered to a central signal station and at that location are interlinked in order to obtain differentiated disturbance or alarm signals.
Automatic fire alarm installations are assigned the task of recognizing at an incipient stage fires or combustion processes, in order to render possible effective fire fighting. One requirement which is placed upon the fire alarms is that they possess high sensitivity in order to be able to use for fire detection purposes traces of the products produced during a combustion process or fire. However, going hand-in-hand with the increase in the sensitivity of the fire alarms is also the undesirable tendency for such fire alarms to trigger false alarms. Therefore, at the central signal station it is frequently difficult to differentiate between an actual alarm and a false alarm.
In order to overcome such shortcomings it has already been proposed to transmit to the central signal station, instead of an alarm signal, a measuring or measurement value which is analogous to a characteristic combustion parameter which is to be measured and determining at the central signal station whether there has been encountered an actual fire or merely a malfunction in the alarm system. This is possible because by comparison of the measuring magnitudes or values of different fire alarms there can be obtained an appreciably more exact determination.
A prerequisite for useful evaluation of the fire alarm signals at the central signal station is, however, that there be clearly ascertained the origin of the signals, i.e. the fire alarms must be identifiable, in other words addressable.
In more recent times there have been developed a number of fire alarm systems wherein identification of the fire alarms is possible and there can be accomplished transmission of the measuring value to the central signal station. However, the expenditure in circuitry is extremely great and the installation is associated with technological difficulties.
One of the primary drawbacks of this technique resides in the fact that for determining the fire alarm address it is necessary to undertake at each fire alarm an individual adjustment or setting. Consequently, there prevails the danger of faulty addressing of the fire alarms and the thus related false identification of the fire alarms.
In order to overcome such drawback there has been proposed in German Pat. No. 2,533,382 a method for transmitting measuring values in a fire alarm system wherein the measuring values which are delivered by individual fire alarms located in a series configuration at the reporting or signaling lines are transmitted in the form of analog signals to a central signal station and at that location are linked in order to obtain differentiated disturbance or alarm reporting signals. At the beginning of one of each interrogation cycle all of the fire alarms are disconnected from the alarm reporting line by a potential change and then again connected in a predetermined sequence in a manner such that each fire alarm, following a time-delay corresponding to its measuring value, additionally connects the momentarily next following fire alarm to the line voltage. Furthermore, at the central signal station there is derived the relevant fire alarm address from the number of preceding increases in the line current and the measuring value is derived from the length of the related switching time-delay.
Yet, this prior art method is associated with three fundamental shortcomings:
Firstly, due to the series arrangement of the fire alarms there are encountered increased costs and time during installation of the fire alarms in order to ensure that the fire alarms are properly connected. Even if there is utilized a two-wire system, nonethless the fire alarm still possesses three terminals and care must be taken to ensure that the inbound wire and the outbound wire are not erroneously interchanged. In contrast to classical installation techniques for two-wire systems this is associated with increased difficulties in installing the fire alarms and also constitutes a source for errors.
Secondly, the number of fire alarms per line is limited by the resistance of the switch which is connected in series.
Thirdly, upon malfunction of a fire alarm or in the event of interruption or short-circuit of the line it is necessary for at least part of the fire alarms of a line to be placed out of operation.